Wet cooling towers are well known in the art and are used in a variety of industries for cooling fluids such as water. The primary use of large, industrial cooling tower systems is to remove the heat absorbed in circulating cooling water systems used in power plants, petroleum refineries, petrochemical and chemical plants, natural gas processing plants and other industrial facilities. The absorbed heat is rejected to the atmosphere by the evaporation of some of the cooling water in mechanical forced-draft or induced draft towers.
Cooling towers are widely used in the petroleum refining industry. Refining of petroleum cannot take place without cooling towers. Refineries process hydrocarbons at high temperatures and pressures. Cooling water is used to control operating temperatures and pressures. The loss of cooling water circulation within a refinery can lead to unstable and dangerous operating conditions requiring an immediate shut down of processing units. Cooling towers have become “mission critical assets” for petroleum refinery production. As demand for high-end products such as automotive and aviation fuel has risen and refining capacity has shrunk, the refineries have incorporated many new processes that extract hydrogen from the lower value by-products and recombined them into the higher value fuels, improving yield. Many of these processes are dependant on cooling to optimize the yield and quality of the product. Over the past decade, many refineries have been adding processes that reform low grade petroleum products into higher grade and more profitable products such as aviation and automotive gasoline. These processes are highly dependent upon the cooling towers to control the process temperatures and pressures that affect the product quality, process yield and safety of the process. In addition, these processes have tapped a great deal of the cooling capacity reserve in the towers leaving some refineries “cooling limited” on hot days and even bottlenecked. With most U.S. refineries operating well above 90% capacity with attractive profit margins, operating the refinery is critical to operating profit and to pay for the process upgrades implemented over the last decade.
Typically, a wet cooling tower system comprises a basin which holds cooling water that is routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams that need to be cooled or condensed, and the absorbed heat warms the circulating water. The warm circulating water is delivered to the top of the cooling tower and trickles downward over fill material inside the tower. The fill material is configured to provide a maximum contact surface, and maximum contact time, between the water and air. As the water trickles downward over the fill material, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. Many wet cooling towers comprise a plurality of cells in which the cooling of water takes place in each cell in accordance with the foregoing technique. Cooling towers are described extensively in the treatise entitled “Cooling Tower Fundamentals”, second edition, 2006, edited by John C. Hensley, published by SPX Cooling Technologies, Inc.
Many cooling towers in use today utilize large fans, as described in the foregoing discussion, to provide the ambient air. The fans are enclosed within a fan cylinder that is located on the fan deck of the cooling tower. Drive systems are used to drive and rotate the fans. The efficiency and production rate of a cooling tower is heavily dependent upon the efficiency of the fan drive system. The duty cycle required of the fan drive system in a cooling tower environment is extreme due to intense humidity, icing conditions, wind shear forces, corrosive water treatment chemicals, and demanding mechanical drive requirements.
One commonly used prior art drive system is a complex, mechanical fan drive system that is similar to the type used in agriculture applications. This type of prior art fan drive system utilizes a motor that drives a drive train. The drive train is coupled to a gearbox, gear-reducer or speed-reducer which is coupled to and drives the fan. This prior art fan drive system is subject to frequent outages, a less-than-desirable MTBF (Mean Time Between Failure), and requires diligent maintenance, such as regular oil changes, in order to operate effectively. Furthermore, prior art gearboxes typically require a separate gear to reverse the rotational direction. One common type of mechanical drive system used in the prior art gearbox-type fan drive utilizes five rotating shafts, eight bearings, three shaft seals (two at high speed), and four gears (two meshes). This drive train absorbs about 3% of the total power. Although this particular prior art fan drive system may have an attractive initial cost, cooling tower end-users found it necessary to purchase additional components such as composite gearbox shafts and couplings in order to prevent breakage of the fan drive components. Many cooling tower end-users also added other options such as low-oil shutdown, anti-reverse clutches and oil bath heaters. Thus, the life cycle cost of the prior art mechanical fan drive system compared to its initial purchase price is not equitable.
In a multi-cell cooling tower, such as the type commonly used in the petroleum industry, there is a fan and prior art mechanical fan drive system associated with each cell. Thus, if there is a shutdown of the mechanical fan drive system associated with a particular cell, then that cell suffers a “cell outage”. A cell outage will result in a decrease in the production of refined petroleum. For example, a “cell outage” lasting for only one day can result in the loss of thousands of refined barrels of petroleum. If numerous cells experience outages lasting more than one day, the production efficiency of the refinery can be significantly degraded. The loss in productivity over a period of time due to the inefficiency of the prior art mechanical fan drive systems can be measured as a percent loss in total tower-cooling potential. As more cell outages occur within a given time frame, the percent loss in total tower-cooling potential will increase. This, in turn, will decrease product output and profitability of the refinery and cause an increase in the cost of the refined product to the end user. It is not uncommon for decreases in the output of petroleum refineries, even if slight, to cause an increase in the price-per-barrel of petroleum and hence, an increase in the cost of gasoline to consumers. The effect of cell outages with respect to the impact of petroleum product prices is described in the report entitled “Refinery Outages: Description and Potential Impact On Petroleum Product Prices”, March 2007, U.S. Department of Energy.
Other types of prior art fan drive systems, such as V-belt drive systems, also exhibit many problems with respect to maintenance, MTBF and performance and do not overcome or eliminate the problems associated with the prior art gearbox-type fan drive systems. One attempt to eliminate the problems associated with the prior art gearbox-type fan drive system is the prior art hydraulically driven fan system. Such a system is described in U.S. Pat. No. 4,955,585 entitled “Hydraulically Driven Fan System For Water Cooling Tower”.
Other types of prior art fan drive systems, such as V-belt drive systems, also exhibit many problems with respect to maintenance, MTBF and performance and do not overcome or eliminate the problems associated with the prior art gearbox-type fan drive systems. One attempt to eliminate the problems associated with the prior art gearbox-type fan drive system was the prior art hydraulically driven fan systems. Such a system is described in U.S. Pat. No. 4,955,585 entitled “Hydraulically Driven fan System For Water Cooling Tower”.
Therefore, in order to prevent supply interruption of the inelastic supply chain of refined petroleum products, the reliability and subsequent performance of cooling towers must be improved and managed as a key asset to refinery production and profit. An efficient and reliable fan drive system is required to maintain a relatively high cooling efficiency and prevent interruptions in production.